The present invention relates to a reverse flow choke valve used in connection with a wellhead, typically a choke valve having a removable insert assembly for an underwater wellhead. The invention also relates to the removable insert assembly component itself, including novel flow trim components.
Choke valves are flow throttling devices which control flow and reduce the pressure of the fluid moving through them. When the pressure is reduced, the velocity of the fluid increases.
The fluid moving through a choke valve can often be severely erosive. For example, a choke valve may be used to control a gas flow containing entrained sand and moving at high pressure and velocity. It follows that a choke valve is a critical piece of wellhead flow control equipment, which must be designed and constructed to cope with an erosive flow.
Choke valves which are used in underwater or sub sea service may be located at great depths, for instance 6000 feet. At such depths, wellhead servicing has to be carried out using an unmanned, remotely operated vehicle, referred to as an “ROV”. Sub-sea choke valves typically are designed with the wear components in an insert assembly which can be removed by remote servicing.
FIG. 1 illustrates a prior art choke valve used in sub sea service. This choke valve is designed to be vertically oriented in use, so that its operating parts can be removed and replaced as a unit using a vertical cable extended from surface. The choke valve includes the following:                A body having a T-shaped arrangement (T on its side) of bores providing a horizontal side inlet, a vertical bottom outlet and a vertical chamber for containing operating components (the inlet and outlet bores have an inverted L shaped configuration);        A generally tubular cartridge vertically positioned in the chamber and extending across the side inlet, the side walls of the cartridge forming a side port connecting with the inlet,        A “flow trim” positioned within the bore of the cartridge and including a stationary tubular cylinder referred to as a nozzle or “cage”, and a vertically oriented, tubular, external sleeve (flow collar) having one closed end, the sleeve being positioned to slide along the cage side wall to throttle the ports. The cage is seated on an internal shoulder formed by the lower end of the cartridge. It extends across the inlet and its bore is vertical, being axially aligned with the outlet. The cage has a plurality of flow ports, spaced around its circumference, extending through its side wall. As shown in FIG. 1, there are generally at least two opposing main ports and at least two opposing smaller ports. The cage flow ports communicate through the cartridge port with the inlet. Fluid enters the cage bore from the horizontal inlet through the flow ports, changes direction within the cage and leaves through the vertical body outlet. In moving through the flow ports, the fluid pressure is reduced and its velocity is increased, thereby increasing the erosiveness of the stream;        A bonnet assembly secured to the cartridge and closing the upper end of the body chamber. The bonnet assembly is also secured to the body by clamp means which can be undone by the ROV, to release the bonnet assembly from the body; and        A stem extending through an opening in the bonnet assembly and connecting with the flow trim sleeve. An actuator (not shown), powered by a hydraulic system operated from surface, can rotate the stern to advance and retract the sleeve, thereby adjusting the open area of the cage flow ports.        
The cartridge and its contained components, as just described, can be referred to as a choke removable insert assembly.
The valve body is formed of softer material, typically steel, while the flow trim components are typically manufactured from a high wear material such as tungsten carbide. The steel body needs to be machined in the course of fabrication and it also has to be able to cope with stresses, and thus is manufactured from a relatively ductile steel. The flow trim however has harder surfaces. Typically the cage of the flow trim is formed of tungsten carbide and a tungsten carbide liner is shrink fitted to line the flow collar. This is important because the flow trim is positioned at the bend of the “L”, where it is exposed to, and temporarily contains, the fluid flow when it is accelerated, is changing direction and is in a turbulent state.
When a sub sea well is first completed, the subterranean formation containing the oil or gas will typically be at sufficient pressure to drive the produced fluid to surface. The well is referred to as a “flowing” well. However, over time the formation pressure diminishes. Eventually it may be desirable to inject water or other fluid into the formation, through one or more wells, to increase its pressure and maintain the formation flowing capability. This requires fluid to be pumped under pressure or “injected” through the choke valve in the opposite or reverse direction. If this is done with the valve shown in FIG. 1, the fluid exits the partly closed ports of the flow trim as high velocity, angled jets that impinge against and erode the steel material of the choke body.
To try to reduce the erosion when a well is converted to reverse flow, well operators have resorted to operating the choke valve with a reduced pressure drop, which is an undesirable restriction. Another alternative is use a multiport cage, for example a 16 port cage, with the ports being arranged circumferentially around the cage, to distribute the wear to the body more uniformly around the cage. However, this is still undesirable since the body, which is retrievable only by retrieving the entire wellhead tree, is still directly exposed to wear.
U.S. Pat. No. 6,648,070 to Cove et al., and owned by the assignee of the present patent application, Master Flo Valve Inc., describes an insert assembly for a sub-sea choke designed for reverse flow with a horizontal cage and collar, but requiring complicated components to actuate these horizontal components.
The present invention is concerned with providing a reverse flow insert assembly which can replace the production insert assembly when the well is to be changed from production to injection, but which is simple in design,